Turkey's Erdogan speaks with Trump amid strained ties over Afrin offensive

Turkish president has spoken by telephone with United States President Donald Trump amid Turkish frustrations with its NATO ally over its support for a Syrian Kurdish militia that Turkey regards as a security threat.

Recep Tayyip Erdogan's spokesman, Ibrahim Kalin, said the two discussed "bilateral ties and regional issues" during Thursday's call, without providing further details.

Turkey on Sunday captured the Syrian Kurdish-held town of Afrin and vowed to expand its offensive to other areas, including to Manbij which is controlled by a US-backed force dominated by Syrian Kurds.

Ankara said Turkish and US officials discussed ways of overcoming differences and have reached an "understanding" over the ousting of the Kurdish militia from Manbij and joint measures to ensure its security.

The US warned Turkey on Monday that it is "deeply concerned" after the Turkish-led assault on the Syrian city of Afrin triggered an exodus of Kurdish civilians.

Turkish forces and Turkish-backed Syrian Arab fighters have over the past 48 hours surged into the city in northwest Syria, once defended by the Kurdish People's Protection Units (YPG) militia.

In eastern Syria, the YPG forms the backbone of the US-backed Syrian Democratic Forces (SDF), the militia which ousted the Islamic State group from its main stronghold of Raqa.

But, while US special forces continue to support the SDF east of the Euphrates river, they have not come to their aid in Afrin, a pocket of autonomous Kurdish-led rule west of the river.

There, Erdogan has unleashed his army and Syrian former rebel fighters to take Afrin from the group he sees as aligned with the outlawed Kurdish PKK group fighting within Turkey.

The fight has embarrassed the US, which wants to maintain ties with both its traditional NATO ally and a regional force that has proved its mettle against IS.